A detector, namely a detector for an electron multiplier and an electron multiplier comprising the electron detector, is disclosed herein.
Electron multipliers can be used to detect the presence of single electrons or other quanta or particles. FIG. 2 shows a cut-through view of an electron multiplier 200 according to the prior art.
Generally, an electron multiplier 200 comprises a chamber 202 defined by side walls 204, a front cover 206 and a rear cover 208. The front cover 206 is transparent to the quanta or particle that the multiplier 200 is designed to detect.
The chamber 202 is held under a vacuum, and contains an electron avalanche mechanism 210. A single electron 212 incident on the front face of the avalanche mechanism 210 is absorbed and causes an electron avalanche 214 to be emitted from the rear face.
Electron multipliers 200 can be used to directly detect incident electrons, quanta or other particles. The avalanche mechanism 210 may initiate an electron avalanche in response to the absorption of an electron, quanta or other particle. For example, an electron avalanche may be initiated in response to absorption of electrons, positively or negatively charged ions or photons.
Alternatively, as shown in FIG. 2, the electron multiplier 200 may comprise a converter 216. The converter 216 absorbs an incident quanta or particle 218 (such as a photon or ion) on its front face, and emits a single electron 212 from its rear face. In this case, the electron multiplier 200 detects an electron which is representative of the incident quanta or particle.
In the example shown in FIG. 2, a single photon 218 is absorbed by the converter 216, which emits a single electron 212. The single electron 212 is then absorbed by the avalanche mechanism 210, which emits an electron avalanche 214.
The electron avalanche 214 is detected by a sensor (not shown in FIG. 2), provided after the avalanche mechanism 210. It is known, for example, to provide the sensor within the chamber 202, having electrical connects provided though the side walls 204 or rear cover 208. This can be used to detect the present of quanta or particles and, for example, count the number of quanta and particles.
The electron multiplier 200 may be provided on its own, such that the outside of the side walls 204, front cover 206 and rear cover 208 is open to atmospheric conditions.
The electron multiplier 200 could also be coupled to a larger vacuum system. In examples where the electron multiplier is coupled to a larger vacuum system, the electron multiplier may be held within the larger vacuum or adjacent a housing forming the larger vacuum (the housing defining the larger vacuum should include a window transparent to the quanta or particle to be detected aligned with the front cover 206 of the electron multiplier).
In another example where the electron multiplier 200 is coupled to a larger vacuum, the chamber 202 may be formed as part of the larger vacuum, with the front cover 206 omitted, and the sidewalls 204 joining directly to the housing defining the larger vacuum.
Any suitable technique can be used to couple the electron multiplier to the larger vacuum.
Avalanche mechanisms 210, converters 216 and electron multipliers 200 of this type are generally well known in the art. However, the electrical connections through the side walls 204 make the sensor difficult to implement. Also, in some applications, it may be important to know the position of detected quanta or particles.
To detect the position of the quanta or particle 218, the electron multiplier 200 should preserve position throughout the multiplier 200. Therefore, at the avalanche mechanism 210 the electron avalanche 214 is emitted from the same (two dimensional) position as where the incident electron 212 is absorbed. Similarly, at the converter 216, the single electron 212 should be emitted from the same (two-dimensional) position as where the incident quanta or particle 218 is absorbed.
Because the avalanche mechanism 210 (and converter) preserve the position of the incident quanta or particle 218, it is possible to determine the position at which the incident quanta or particle 218 over the face of the electron multiplier 200 by determining the position of the electron avalanche 214.
The electron avalanche 214 is first collected for a short time inside the vacuum via a high-resistance collector. The electron avalanche is then read out capacitively as an image charge, through the rear cover 208. This is done via a low-resistance anode layer, outside the chamber 202.
The anode layer comprises a plurality of contact regions. The image charge spreads as it capacitively couples through the counter-substrate, so the image charge is incident on more than one of the contact regions. The position of the incident electron can be determined by processing the charges induced on the different contact regions (also known as “centroiding”).
U.S. Pat. No. 5,686,721 is one example of a sensor that can be used for determining the position of an incident electron.
Two events (detection of quanta 218) will be considered simultaneous, or near-simultaneous, if they result in image charges incident on one or more of the contact regions at the same time. If simultaneous or near-simultaneous events induce image charges in one or more of the same contact regions, centroiding is not able to determine the two separate events. Instead, the two induced charges are interpreted as a single event, with a position derived from treating the two induced image charges as a single image charge. Therefore, centroiding is limited to serial events (i.e. one after another) for events in close proximity and cannot be used to detect simultaneous or near-simultaneous events.
According to a first aspect of the disclosure, there is provided a detector for an electron multiplier, the detector comprising: a substrate comprising a dielectric material, the substrate having a first face and an opposing second face; a charge collector provided adjacent the first face of the substrate; an anode within the substrate, the anode spaced from first face, such that the anode is capacitively coupled to the charge collector, so that charge incident on the charge collector generates an image charge on the anode; and a conduit contact, coupled to the anode and passing through the substrate to the second face of the substrate layer.
Burying the anode within the substrate means that the image charge spreads less before being detected, without having to provide electrical connections all the way through the rear wall. As such, there is less chance of two simultaneous or near-simultaneous events occurring in close proximity to one another inducing image charges on a common contact of the anode, improving the resolution of the detector.
The substrate may comprise a dielectric layer forming the first face. The anode may be provided within the dielectric layer or at the edge of the dielectric layer.
The substrate may comprise a ceramic layer forming the second face. The anode may be provided at the interface between the ceramic layer and the dielectric layer.
The spacing between the first face of the substrate and the anode may be at most 1 mm.
The conduit contact may comprise a via portion passing through the substrate and a planar portion, on the second face of the substrate.
The detector may further comprise circuitry, coupled to the conduit contact. The circuitry may be arranged to read-out the image charge induced in the anode and may comprise a processor arranged to determine the position of an electron avalanche incident on the charge collector, based on the detected charge.
The anode may comprise a plurality of electrically isolated anode plates. Each anode plate may be coupled to a separate conduit contact and the circuitry may be arranged to separately readout the image charge from each anode plate.
The circuitry may be arranged to readout the charge induced in at least two anode plates simultaneously.
The processor may be arranged to compare the image charge simultaneously induced in neighboring anode plates and to determine the position of an electron avalanche incident on the charge collector based on the comparison.
The charge collector may be a resistive layer. The resistive layer may have a sheet resistance of at least 250 kohm/square. The resistive layer may have a sheet resistance of 500 kohm/square; 750 kohm/square or 1 Mohm/square.
According to a second aspect of the disclosure, there is provided an electron multiplier comprising means for initiating an electron avalanche in response to a single detected electron, the electron avalanche emitted in the same location as the single electron is detected; and a detector, according to the first aspect, for detecting the electron avalanche and determining the position of the single electron from the detected position of the electron avalanche.
The electron multiplier may comprise a measurement chamber holding the means of initiating an electron avalanche in a vacuum. The chamber may be defined by a housing, the substrate forming at least part of the housing, such that the charge collector is held in the vacuum and the second face of the substrate is held outside the vacuum.
The electron multiplier may further comprise an electron converter having a first face and an opposing second face, the electron converter, arranged to receive a photon or other quanta at a first location on the first face, and, in response to a received photon or quanta, emit an electron from the first location on the second face, the electron emitted towards the means for initiating an electron avalanche.